Conventional medical electronic thermometers use a sensing element, e.g., thermocouple, thermistor or diode, whose electrical characteristics change to change voltage or current circuit output as a function of the ambient temperature of the environment in which sensing element is immersed. The electrical signal provided by such a sensing element or temperature transducer is then measured as relating to the ambient temperature. As the transducers do not heat up to ambient temperature instantaneously, the output of these temperature transducers will lag in instantaneous actual temperature being measured as a function of the heat transfer curve of the individual transducer.
Medical electronic thermometers having anticipation or predictive components which enable the actual ambient temperature being measured to be calculated in advance of a true indication of this temperature from the transducer have been recently developed. These thermometers predict the final temperature in a number of ways. Georgi, U.S. Pat. No. 3,702,076, recognizing that the output of the temperature transducer follows the decaying exponential curve describing the heat transfer characteristics of the transducer, predicts the final value or the asymptote which the curve approaches by adding a fixed increment to the transducer signal at a fixed time after the transducer has been introduced to the ambient temperature being measured determined by the time between error pulses which controls the balance of a bridge circuit.
Kauffeld, U.S. Pat. No. 3,872,726, predicts the asymptote to the decaying exponential curve, i.e., the steady state or final transducer temperature, by looking for a specific rate of change in the response curve which is then correlated to the point where the transducer output is a predetermined increment below final value. This point being determined, the fixed increment is added to transducer signal to give the final or steady state value in advance of it actually being reached.
Goldstein, U.S. Pat. No. 3,978,325, predicts the asymptote to the curve, the final value which the transducer will produce, by mathematically computing it by solving the equation for the curve having obtained two points on the curve.
The Georgi and the Kauffeld predictive circuits require a relatively long period before they are able to make a prediction. As much as 90% of ultimate transducer temperature excursion must occur before a prediction is available. The Goldstein circuit must also wait a relatively long period in order to obtain two sample points. Sample points which are too close together require exacting arithmetic calculation implementation and too large a computational unit for a medical electronic thermometer.
This inventor, with a previous invention U.S. Pat. No. 3,972,237, also teaches a prediction circuit. This circuit recognizes the fact that the transducer heat transfer curve of the temperature transducer and therefore the transducer output signal response is an exponential curve which approaches an asymptote similarly to the operation of a system which approximates a critically-damped control system following a first derivative curve. By generating a second derivative error curve which can be added to the first derivative curve the steady state approximation which translates into the final output value of the temperature transducer may be obtained in advance. Like the other circuits referenced above, this predictive circuit will yield a more accurate prediction only when precise circuit component values are used and only after a substantial derivative history has been sampled.